US naval forces in the Red Sea this month boarded a freighter suspected of delivering Iranian weapons to Houthi rebels in Yemen, American military officials said.

The destroyer USS Sterett’s search of the Panamanian-flagged Saisaban on April 1 came up empty. But the officials said it marked the US Navy’s first boarding operation in an expanding campaign to ensure Iran doesn’t supply game-changing weapons such as surface-to-air missiles that would threaten Saudi-led airstrikes on the Houthis, The Wall Street Journal reported on Monday.

The airstrikes began two weeks ago after Houthi rebels, who have taken over the capital and overrun much of the country, forced American-backed President Abed Rabbo Mansour Hadi to flee the country.

US and Saudi officials said Tehran has been providing arms, weapons, training and funding for the Houthis for years—allegations Iran denies. A senior defense official said the US knows Tehran is trying to supply the militant group with surface-to-air missiles.

Since the Red Sea search, the US military has stepped up its surveillance in the region so it can keep a closer eye on what Iran and the Houthis are doing to turn the tide in their favor, the US officials said.